Avian defective leukemia viruses (DLV) have been useful in defining some of the mechanisms in viral oncogenesis. Our laboratory has been extensively engaged in the study of different strains of acute leukemia viruses and their interaction with avian hemopoiesis. Our goal is to continue the systematic characterization of the target cell for avian retroviruses and its relationship with cellular differentiation. We have described the interesting findings of spontaneous differentiation of leukemic cells after infection of embryonic cells with avian erythroblastosis virus (AEV). This proposal will include a detailed analysis of the nature of the embryonic target cell and the type of hemoglobin expressed before and after differentiation. A mast-like cell has been isolated from leukemic Japanese quail infected with AEV. A careful study will be directed to the important question of how erythroblastosis virus induces a different type of leukemia in another host. Moreover, this proposal will attempt to examine whether different lineages are involved in the leukemic process.